The pharmaceutical industry, the cosmetic industry and other industries have long been concerned with the formation of gel products that have viscosity stability. In particular, these industries have spent significant resources in developing gel products that maintain substantially the same viscosity over extended periods of time.
A great many different viscosity or gelling agents have been discovered and used in a wide variety of products. These viscosity agents include, without limitation, carboxyvinyl polymers, gums (e.g., xanthan gum), sodium hyaluronate, cellulosics (e.g., hydroxyethyl cellulose (HEC) and hydroxypropylmethyl cellulose (HPMC)), polyethylene glycol and the like. For nearly all of these viscosity agents, it has been found that free radicals tend to inhibit their ability to maintain viscosity of their respective compositions. In particular, free radicals tend to initiate oxidation reactions, which ultimately release additional free radicals, which then cause further oxidation reactions. This oxidation reaction cycle then degrades the viscosity agents and degrades the ability of the viscosity agents to maintain viscosity or the compositions.
To combat the degradation of viscosity agents through oxidation reactions, the various industries have focused upon the use of anti-oxidants that act as free radical scavengers. These anti-oxidants act to lower the number of free radicals in the composition and, in turn, lower the number of oxidation reactions occurring in the composition.
Anti-oxidants are generally effective in limiting oxidation reactions and are effective for aiding in maintaining viscosity stability of the overall composition. However, it has also been more recently discovered that many of the anti-oxidants themselves tend to lower viscosity of at least some of compositions albeit typically significantly less than would be caused by the oxidation reactions, which the anti-oxidants prevent.
Cellulosic ether viscosity enhancers (e.g., HPMC and HEC) are susceptible to degradation since these cellulosics typically include peroxy free radicals as impurities. Of course, these peroxy free radicals can initiate oxidation reactions, which can degrade these viscosity agents. However, it has been surprisingly discovered by the inventors of the present invention that, in particular compositions, antioxidants such as ion chelators, particularly ethylenediaminetetraacetic acid (EDTA), tend to significantly reduce viscosity of cellulosic ethers, particularly HEC, over time. Further, it has been discovered that, in these particular compositions, cellulosic ethers, particularly HEC, tends to maintain its viscosity quite well in the absence of any antioxidant.